As soon as I saw the thread title I was going to ask for a photo, but you already gave us one
she is gorgeous!
Though my own experience of mules is limited to a handful here and there, the bumping seems common and would logically come from the donkey side correct.
The difficulty is in reading it because it can have several meanings. It can be territorial sure, but also it helps the donkey to develop balance and is a common form of communication in the group.
What is her reaction if you interrupt her? Ie if you wait until you know she is coming from behind then suddenly spin 180 and stamp the ground say?
In terms of handling I would treat her as a horse but just remember they think a little differently, and also remember that horses tend to push with their shoulder whereas donkeys are much more head on. (like humans)